1. Field of the Invention
The present invention relates to a plastic control device fabricated on the model of the plastic behavior of synapses More particularly, the present invention relates to an artificial plastic control device which simulates the plasticity of synaptic transmission associated with learning and memory in the living body.
2. Description of the Related Art
Information processing activities such as learning, memory, reasoning and like are very natural to organisms but are extremely difficult for existing computers which consist primarily of silicon devices because the computers presently available perform functions by simple devices according to complex software prepared therefor, whereas organisms include a system which itself has complex software incorporated therein. Accordingly, highly functional devices, if fabricated, will make it possible to use the inherent information processing process of organisms for engineering applications.
One of the distinct differences between the brain and the computer is memory. The memories (as means) in the computer are present independently of one another, and the capacity of the memories is determined in proportion to the number of devices concerned. On the other hand, the memories in the brain are present as associated with one another, and there is no proportional relation between the capacity of the memories and the number of devices. When a new memory is prepared, the memory is assigned a new address in the case of the computer, whereas with the brain, it is thought that the memory is assigned part of the existing network of devices.
In the living body, memory as an activity is observed as a state of the junction between neurons, i.e., in the form of "synaptic plasticity". FIG. 6 is a diagram showing a synapse which is the junction between two neurons, and main factors giving rise to synaptic plasticity. A typical example of such "synaptic plasticity" is seen at the junction between the perforant path and the granule cell of the rabbit hippocampus; it has been observed that the transmission efficiency increases as stimulation is given more frequently (T. V. P. Bliss et al., J. Physiol. 232(1973)331-).
From this viewpoint, on the other hand, reports are recently made on artificial presynaptic models. L. L. Miller et al, coated an electrode with a polymer containing a neurotransmitter, such as dopamine or glutamic acid, stating that the release of the neurotransmitter was controllable electrochemically (L. L. Miller et al., J. Am. Chem. Soc., 104(1982)5242-5244; ibid., 105 (1983)5271-5277; ibid., 105(1983)5278-5284). Another report says that when the polymer was prepared by electropolymerization (with use of a neurotransmitter as the dopant to be incorporated into the polymer during polymerization), the release of the neurotransmitter was similarly controllable electrochemically (R. L. Blankespoor et al., J. Chem. Soc., Chem. Commun., 1985) 90-92; Hiroaki Shinohara et al., NIPPON KAGAKU KAISHI No. 3(1986) 465-469). Further Shimidzu et al. used the combination of an electropolymerized film having anion capturing ability and an electropolymerized film having cation capturing ability as electropolymerized polymer films of improved function, suggesting the feasibility of a novel electrochemical deionization system as a substitute for conventional ion-exchange resins (T. Shimidzu et al., J. Electroanal. Chem., 251(1988)323-337).
The foregoing reports on various artificial presynaptic models all simulate only the release of neurotransmitters but do not disclose engineering applications of function of actual synapses, nor do they show such applications of plastic control.
We have already developed a control device which simulates the plasticity of synaptic transmission and in which the input signal transmission efficiency varies with time (plastic behavior) (Unexamined Japanese Patent Publication SHO 63-200396). However, the device has the problems of: (1) being irreversible, (2) using a stimulation signal, which is identical with the input, for varying the weight, and (3) necessitating a complex treatment such as potential sweeping.
The present invention, which has been accomplished in view of the above situation, provides an epoch-making plastic control device which exhibits the plastic behavior of synaptic transmission as described above and which comprises a simple circuit to realize reversibility.
Directing attention to electronically conducting polymers prepared by electropolymerization, we have carried out intensive research on the fabrication of devices which vary in signal transmission efficiency with time as stated above, using such polymers.
In the case where the charge carrier is electron is common electronically conducting polymers (as used as dry conductors), the conduction velocity is difficult to control, so that the polymer is difficult to use in plastic control devices. However, in the case of ion conductors wherein the charge carrier is an ion, it is possible to electrochemically control the release of the ion, i.e., dopant, into an electrolyte and incorporation of the dopant into the conducting polymer. On the other hand, it is difficult to provide a reversible plastic control device by a system wherein the variation with time or electrolytic current due to release or incorporation of the dopant during application of voltage is utilized directly for plastic control.